1. Field of the Invention
The present invention relates generally to gas bearing turbocompressors. More particularly, the invention concerns turbocompressors embodying gas bearing wherein the average static pressure in the compressor housing differs substantially from the average static pressure in the expander housing.
2. Discussion of the Prior Art
Gas bearings have been in use for many years in applications wherein a shaft rotates at very high speeds within a stationary journal. The well-known advantages of these devices are very low frictional losses and lack of a complex oil lubricating system, usually comprising an oil reservoir, oil pump, filters, coolers pressure gauges, valves and related hardware.
In all fluid-supported bearings, the hydrodynamic action which generates dynamic pressures within the space between the shaft and the journal is caused by the motion of a viscous fluid (whether oil or gas) which is forced to flow in a gap of varying width by the relative motion of the shaft within the journal. In the simplest case of a cylindrical shaft rotating in a cylindrical journal, the variation in the width of the gap is the consequence of an eccentricity of the shaft in the journal, such as may be caused by the weight of the shaft or other externally applied forces. This simplest bearing is stable under certain conditions, but becomes unstable when the combination of applied forces and rotational speed exceeds a certain value. Furthermore, the load-carrying capacity of the simple gas bearing is not very high.
Considerable ingenuity has been exercised in introducing modifications to the cylindrical surface of the journal to achieve a more effective variation of gap width for smaller values of eccentricity. These prior art endeavors have resulted in a variety of shapes being proposed, as for example, three-lobe bearings, lemon-shaped bearings, half lemon-shaped bearings, and displaced bearings.
A substantial improvement over prior art fluid-bearing devices is disclosed in U.S. Pat. No. 4,808,070 issued to the present inventor. The device described in this patent includes a novel fluid bearing which retains all of the advantages of prior art gas bearings, but obviates the difficulty and expense of manufacture of such devices by substantially relaxing the dimensional tolerances required. This patent also describes a fluid bearing which is intrinsically stable under widely varying operating conditions. Because of the relevance of U.S. Pat. No. 4,808,070 to a complete understanding of the present invention, this patent is hereby incorporated by reference as though fully set forth herein.
The thrust of the present invention is directed toward the solution to problems resulting from the use of gas bearings in turbocompressors wherein the average static pressure in the compressor housing differs from the average static pressure in the expander housing or vice versa. By way of example, such a condition is found in the upper stages of a multi-stage chain of compressors arranged in series driven by expanders arranged in parallel. In such cases the rotating assembly consisting of the gas-bearing shaft, the expander wheel, the compressor wheel and any thrust bearings, is subjected to an axial force (thrust) equal to the difference of pressure at the two ends of the shaft times the area of the shaft cross section. Since gas bearings in a thrust-resisting configuration are not as stiff as oil bearings in similar service, it is necessary to provide equalizing means for reducing the net resulting thrust to as close to zero as is possible without otherwise compromising the performance of the turbocompressor. A primary object of the present invention is to provide such equalizing means.